add: c++ extension to cpp known extensions 🔨

[Ahattalla]

Description

I've added c++ extension to known extensions of cpp, to allow recognition of files of this extension as c plus plus ones

Related Issues

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Checklist

• I read and understood the contributing guide as well as the code of conduct
• The issues this PR addresses are related to each other
• My changes generate no new warnings
• My code builds and runs on my machine
• My changes are all related to the related issue above
• I documented my code

Screenshots

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[thecoolwinter]

LGTM, thanks!